Sensing circuitry, such as circuitry used to sense temperature, typically has a sensing element and excitation circuitry. The maximum resolution of such sensing circuitry depends on the physical noise limits of the sensing element, and on limitations of the excitation circuitry.
Integrated silicon temperature sensors are currently under investigation because it would be desirable to have an integrated sensor manufactured on the same substrate as signal processing circuitry. Using a single chip is advantageous because of the small size and because the circuitry can perform on-chip processing prior to transmission of data off-chip. Most integrated temperature sensors use a p-n diode as the sensing element, since these diodes can be easily manufactured in a standard bipolar or CMOS process, while it is difficult to manufacture good thermistor-type materials and circuitry on the same wafer using standard processing techniques. These diodes are also preferable because they exhibit a temperature sensitivity of about -2 mV/.degree. C., which is larger than most other circuit elements.
One measuring approach is to provide a known current source and a diode in series, and to measure the voltage across the diode. A relationship can be derived in which the diode voltage is expressed as a number of parameters which are not sensitive to temperature and the logarithm of the temperature. This straightforward approach has two problems: first, the inverse relationship is complex and nonlinear, thus complicating the signal processing as well as the sensor calibration; and second, there is a large turn-on voltage of the diode (about 0.7 V bias voltage), relative to the sensitivity of -2 mV/.degree.C.
These problems can be addressed by using two diodes, each driven by a different current source. The output signal is the difference between the voltages developed across the diodes. The turn-on voltages of the two diodes effectively cancel each other out, and the resulting relationship is linear between the output voltage and the temperature. This approach has other drawbacks. Due to mismatching of the diodes, a gain error can result--although this error can be quantified and eliminating by using a two point calibration. More significantly, an error can be caused by mismatched current sources and temperature coefficients. These errors can produce a nonlinearity in the measurement that can be significant.
To address the problem of mismatch errors, another approach is to use a square wave current source which provides current at two different levels. This single current source generates two different diode voltages that can be subtracted. When the current source output is at a first current level, a first output voltage is sampled and stored. When the current source is switched to a second current level, a second output voltage is sampled and subtracted from the first voltage. As a result, the measurement is independent of the sensor and the current source. In this case however, power supply noise is coupled directly into the measured signal. This situation is different from a differential circuit where such noise appears as a common mode signal and is rejected.
Other efforts have focused on techniques such as controlling currents independently, and use of temperature to frequency conversion with a thermistor to improve the measurement. As a result of these efforts, the best resolution known for an integrated circuit sensor without a thermistor has been about 0.01.degree. C., which is nearly three times worse than the resolution attained with discrete, thermistor-based systems.